Sams Teach Yourself JavaScript in 24 Hours

By Michael Moncur

• Table of Contents
  • Copyright
  • About the Author
  • Acknowledgments
  • We Want to Hear from You!
  • Reader Services
  • Introduction
  • Part I: Getting Started
  • Hour 1. Understanding JavaScript
  • Hour 2. Creating a Simple Script
  • Hour 3. How JavaScript Programs Work
  • Part II: Learning JavaScript Basics
  • Hour 4. Using Functions and Variables
    • Using Functions
    • Understanding Expressions and Operators
    • Using Variables
    • Data Types in JavaScript
    • Converting Between Data Types
    • Workshop: Storing User Data in Variables
    • Summary
    • Q&A
    • Quiz
    • Exercises
  • Hour 5. Using Strings and Arrays
  • Hour 6. Testing and Comparing Values
  • Hour 7. Repeating Yourself: Using Loops
  • Hour 8. Using Math and Date Functions
  • Part III: The Document Object Model (DOM)
  • Hour 9. Working with the Document Object Model
  • Hour 10. Responding to Events
  • Hour 11. Using Windows and Frames
  • Hour 12. Getting Data with Forms
  • Hour 13. Using Graphics and Animation
  • Part IV: Moving on to Advanced JavaScript Features
  • Hour 14. Creating Cross-Browser Scripts
  • Hour 15. Creating Custom Objects
  • Hour 16. Working with Sounds and Plug-Ins
  • Hour 17. Debugging JavaScript Applications
  • Part V: Working with Dynamic HTML (DHTML)
  • Hour 18. Working with Style Sheets
  • Hour 19. Using Dynamic HTML (DHTML)
  • Hour 20. Using Advanced DOM Features
  • Part VI: Putting It All Together
  • Hour 21. Improving a Web Page with JavaScript
  • Hour 22. Creating a JavaScript Game
  • Hour 23. Creating DHTML Applications
  • Hour 24. JavaScript Tips and Tricks
  • Part VII: Appendices
  • Appendix A. Other JavaScript Resources
  • Appendix B. Tools for JavaScript Developers
  • Appendix C. Glossary
  • Appendix D. JavaScript Quick Reference
  • Appendix E. DOM Quick Reference

Converting Between Data Types

JavaScript handles conversions between data types for you whenever it can. For example, you've already used statements like this:

document.write("The total is " + total);

This statement prints out a message such as "The total is 40." Since the document.write function works with strings, the JavaScript interpreter automatically converts any non-strings in the expression (in this case, the value of total) to strings before performing the function.

This works equally well with floating-point and boolean values. However, there are some situations where it won't work. For example, the following statement will work fine if the value of total is 40:

average = total / 3;

However, the total variable could also contain a string; in this case, the statement above would result in an error.

In some situations, you may end up with a string containing a number, and need to convert it to a regular numeric variable. JavaScript includes two functions for this purpose:

• parseInt() converts a string to an integer number.
• parseFloat() converts a string to a floating-point number.

Both of these functions will read a number from the beginning of the string, and return a numeric version. For example, these statements convert the string "30 angry polar bears" to a number:

stringvar = "30 angry polar bears";
numvar = parseInt(stringvar);

After these statements execute, the numvar variable contains the number 30. The non-numeric portion of the string is ignored.

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